Image sensors with multipart diffractive lenses

ABSTRACT

An image sensor may include an array of imaging pixels. Each imaging pixel may have a photosensitive area that is covered by a respective multipart diffractive lens to focus light onto the photosensitive area. The multipart diffractive lenses may have multiple portions with different indices of refraction. The portions of the diffractive lenses closer to the center of the diffractive lenses may have higher indices of refraction to focus light. Alternatively, the portions of the diffractive lenses closer to the center of the diffractive lenses may have lower indices of refraction to defocus light. The multipart diffractive lenses may have stacked layers with the same refractive indices but different widths.

This application is a continuation of U.S. patent application Ser. No.16/125,479, filed Sep. 7, 2018, which is hereby incorporated byreference herein in its entirety. This application claims the benefit ofand claims priority to U.S. patent application Ser. No. 16/125,479,filed Sep. 7, 2018.

BACKGROUND

This relates generally to image sensors and, more particularly, to imagesensors having lenses to focus light.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an electronic device is provided with an array of imagepixels arranged in pixel rows and pixel columns. Each image pixel in thearray includes a photodiode that is coupled to a floating diffusionregion via a transfer gate. Each pixel receives incident photons (light)and converts the photons into electrical signals. Column circuitry iscoupled to each pixel column for reading out pixel signals from theimage pixels. Image sensors are sometimes designed to provide images toelectronic devices using a Joint Photographic Experts Group (JPEG)format.

Conventional image sensors sometimes include a color filter element anda microlens above each pixel. The microlenses of conventional imagesensors typically have curved surfaces and use refraction to focus lighton an underlying photodiode. However, these types of microlenses mayallow peripheral light to pass through the microlenses without beingfocused, leading to optical cross-talk.

It would therefore be desirable to provide improved lenses for imagesensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative electronic device thatmay include an image sensor in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals from the pixel array inaccordance with an embodiment.

FIG. 3A is a cross-sectional side view of an illustrative focusingdiffractive lens with a greater index of refraction than the surroundingmedium in accordance with an embodiment.

FIG. 3B is a cross-sectional side view of an illustrative defocusingdiffractive lens with a lower index of refraction than the surroundingmedium in accordance with an embodiment.

FIGS. 4A and 4B are cross-sectional side views of illustrativediffractive lenses showing how the thickness of the diffractive lens maybe adjusted to change the response to incident light in accordance withan embodiment.

FIG. 5A is a cross-sectional side view of an illustrative multipartfocusing diffractive lens with two portions having greater indices ofrefraction than the surrounding medium in accordance with an embodiment.

FIG. 5B is a cross-sectional side view of an illustrative multipartdefocusing diffractive lens with two portions having lower indices ofrefraction than the surrounding medium in accordance with an embodiment.

FIG. 5C is a cross-sectional side view of an illustrative multipartfocusing diffractive lens with three portions having greater indices ofrefraction than the surrounding medium in accordance with an embodiment.

FIG. 5D is a cross-sectional side view of an illustrative asymmetricmultipart focusing diffractive lens with two portions having greaterindices of refraction than the surrounding medium in accordance with anembodiment.

FIG. 5E is a cross-sectional side view of an illustrative asymmetricmultipart defocusing diffractive lens with two portions having lowerindices of refraction than the surrounding medium in accordance with anembodiment.

FIG. 6 is a cross-sectional side view of an illustrative image sensorwith multipart diffractive lenses formed over the photosensitive area ofeach pixel in accordance with an embodiment.

FIGS. 7A-7E are top views of illustrative multipart diffractive lensesshowing different shapes for the multipart diffractive lenses inaccordance with an embodiment.

FIG. 8A is a cross-sectional side view of an illustrative multipartfocusing diffractive lens with two stacked layers having greater indicesof refraction than the surrounding medium in accordance with anembodiment.

FIG. 8B is a cross-sectional side view of an illustrative multipartdefocusing diffractive lens with two stacked layers having lower indicesof refraction than the surrounding medium in accordance with anembodiment.

FIG. 9A is a cross-sectional side view of an illustrative asymmetricmultipart focusing diffractive lens with two stacked layers havinggreater indices of refraction than the surrounding medium in accordancewith an embodiment.

FIG. 9B is a cross-sectional side view of an illustrative asymmetricmultipart defocusing diffractive lens with two stacked layers havinglower indices of refraction than the surrounding medium in accordancewith an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors with pixelsthat include diffractive lenses. An electronic device with a digitalcamera module is shown in FIG. 1. Electronic device 10 may be a digitalcamera, a computer, a cellular telephone, a medical device, or otherelectronic device. Camera module 12 (sometimes referred to as an imagingdevice) may include image sensor 16 and one or more lenses 29. Duringoperation, lenses 29 (sometimes referred to as optics 29) focus lightonto image sensor 16. Image sensor 16 includes photosensitive elements(e.g., pixels) that convert the light into digital data. Image sensorsmay have any number of pixels (e.g., hundreds, thousands, millions, ormore). A typical image sensor may, for example, have millions of pixels(e.g., megapixels). As examples, image sensor 16 may include biascircuitry (e.g., source follower load circuits), sample and holdcircuitry, correlated double sampling (CDS) circuitry, amplifiercircuitry, analog-to-digital (ADC) converter circuitry, data outputcircuitry, memory (e.g., buffer circuitry), address circuitry, etc.

Still and video image data from image sensor 16 may be provided to imageprocessing and data formatting circuitry 14 via path 27. Imageprocessing and data formatting circuitry 14 may be used to perform imageprocessing functions such as automatic focusing functions, depthsensing, data formatting, adjusting white balance and exposure,implementing video image stabilization, face detection, etc. Forexample, during automatic focusing operations, image processing and dataformatting circuitry 14 may process data gathered by phase detectionpixels in image sensor 16 to determine the magnitude and direction oflens movement (e.g., movement of lens 29) needed to bring an object ofinterest into focus.

Image processing and data formatting circuitry 14 may also be used tocompress raw camera image files if desired (e.g., to Joint PhotographicExperts Group or JPEG format). In a typical arrangement, which issometimes referred to as a system on chip (SOC) arrangement, camerasensor 16 and image processing and data formatting circuitry 14 areimplemented on a common integrated circuit. The use of a singleintegrated circuit to implement camera sensor 16 and image processingand data formatting circuitry 14 can help to reduce costs. This is,however, merely illustrative. If desired, camera sensor 14 and imageprocessing and data formatting circuitry 14 may be implemented usingseparate integrated circuits. If desired, camera sensor 16 and imageprocessing circuitry 14 may be formed on separate semiconductorsubstrates. For example, camera sensor 16 and image processing circuitry14 may be formed on separate substrates that have been stacked.

Camera module 12 may convey acquired image data to host subsystems 19over path 18 (e.g., image processing and data formatting circuitry 14may convey image data to subsystems 19). Electronic device 10 typicallyprovides a user with numerous high-level functions. In a computer oradvanced cellular telephone, for example, a user may be provided withthe ability to run user applications. To implement these functions, hostsubsystem 19 of electronic device 10 may include storage and processingcircuitry 17 and input-output devices 21 such as keypads, input-outputports, joysticks, and displays. Storage and processing circuitry 17 mayinclude volatile and nonvolatile memory (e.g., random-access memory,flash memory, hard drives, solid state drives, etc.). Storage andprocessing circuitry 17 may also include microprocessors,microcontrollers, digital signal processors, application specificintegrated circuits, or other processing circuits.

As shown in FIG. 2, image sensor 16 may include pixel array 20containing image sensor pixels 22 arranged in rows and columns(sometimes referred to herein as image pixels or pixels) and control andprocessing circuitry 24 (which may include, for example, image signalprocessing circuitry). Array 20 may contain, for example, hundreds orthousands of rows and columns of image sensor pixels 22. Controlcircuitry 24 may be coupled to row control circuitry 26 and imagereadout circuitry 28 (sometimes referred to as column control circuitry,readout circuitry, processing circuitry, or column decoder circuitry).Pixel array 20, control and processing circuitry 24, row controlcircuitry 26, and image readout circuitry 28 may be formed on asubstrate 23. If desired, some or all of the components of image sensor16 may instead be formed on substrates other than substrate 23, whichmay be connected to substrate 23, for instance, through wire bonding orflip-chip bonding.

Row control circuitry 26 may receive row addresses from controlcircuitry 24 and supply corresponding row control signals such as reset,row-select, charge transfer, dual conversion gain, and readout controlsignals to pixels 22 over row control paths 30. One or more conductivelines such as column lines 32 may be coupled to each column of pixels 22in array 20. Column lines 32 may be used for reading out image signalsfrom pixels 22 and for supplying bias signals (e.g., bias currents orbias voltages) to pixels 22. If desired, during pixel readoutoperations, a pixel row in array 20 may be selected using row controlcircuitry 26 and image signals generated by image pixels 22 in thatpixel row can be read out along column lines 32.

Image readout circuitry 28 may receive image signals (e.g., analog pixelvalues generated by pixels 22) over column lines 32. Image readoutcircuitry 28 may include sample-and-hold circuitry for sampling andtemporarily storing image signals read out from array 20, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 20 for operating pixels 22 and for readingout image signals from pixels 22. ADC circuitry in readout circuitry 28may convert analog pixel values received from array 20 intocorresponding digital pixel values (sometimes referred to as digitalimage data or digital pixel data). Image readout circuitry 28 may supplydigital pixel data to control and processing circuitry 24 over path 25for pixels in one or more pixel columns.

FIGS. 3A and 3B are cross-sectional side views of illustrativediffractive lenses that may be used in image sensors. As shown in FIG.3A, a diffractive lens 42 may be formed in a surrounding medium 44. Thesurrounding material 44 may be formed from a first material that has afirst index of refraction (n1). Diffractive lens 42 may be formed from asecond material that has a second index of refraction (n2). In theexample of FIG. 3A, the index of refraction of the lens may be greaterthan the index of refraction of the surrounding material (i.e., n2>n1).This results in incident light being focused towards a focal point. Inthis arrangement, diffractive lens 42 acts as a convex lens.

Lens 42 may be transparent to incident light. Therefore, some light maypass through the lens without being focused. For example, incident light46-1 may pass through the center of diffractive lens 42. Thecorresponding light 46-2 on the other side of the diffractive lens maytravel in the same direction as incident light 46-1. In contrast,incident light at the edge of diffractive lens 42 may be redirected dueto diffraction. For example, incident light 46-3 may pass by the edge ofdiffractive lens 42. The light may be redirected such that the outputlight 46-4 travels at an angle 48 relative to the incident light 46-3.In other words, the diffractive lens redirects the light at the edge ofthe lens using diffraction.

Diffraction occurs when a wave (such as light) encounters an obstacle.When light passes around the edge of an object, it will be bent orredirected such that the direction of the original incident lightchanges. The amount and direction of bending depends on numerousfactors. In an imaging sensor, diffraction of light can be used (withdiffractive lenses) to redirect incident light in desired ways (i.e.,focusing incident light on photodiodes to mitigate optical cross-talk).

In the example of FIG. 3A, diffractive lens 42 has an index ofrefraction greater than the index of refraction of the surroundingmedium 44. This causes incident light to be focused towards a focalpoint. However, this example is merely illustrative and otherembodiments may be used.

As shown in FIG. 3B, a diffractive lens 50 may be formed in asurrounding medium 52. The surrounding material 52 may be formed from afirst material that has a first index of refraction (n1). Diffractivelens 50 may be formed from a third material that has a third index ofrefraction (n3). In the example of FIG. 3B, the index of refraction ofthe lens may be less than the index of refraction of the surroundingmaterial (i.e., n1>n3). This results in incident light 46 beingdefocused. In this arrangement, diffractive lens 50 acts as a concavelens.

Lens 50 may be transparent to incident light. Therefore, some light maypass through the lens without being focused. For example, incident light46-1 may pass through the center of diffractive lens 50. Thecorresponding light 46-2 on the other side of the diffractive lens maytravel in the same direction as incident light 46-1. In contrast,incident light at the edge of diffractive lens 50 may be redirected dueto diffraction. For example, incident light 46-3 may pass by the edge ofdiffractive lens 50. The light may be redirected such that the outputlight 46-4 travels at an angle 54 relative to the incident light 46-3.In other words, the diffractive lens redirects the light at the edge ofthe lens using diffraction.

In addition to the refractive indices of the diffractive lens and thesurrounding material, the thickness of the diffractive lens may alsoaffect the response of incident light to the diffractive lens. FIGS. 4Aand 4B show illustrative diffractive lenses used to focus incident light(as in FIG. 3A, for example). As shown in FIG. 4A, a diffractive lens 42may be formed in a surrounding medium 44. The surrounding material 44may be formed from a first material that has a first index of refraction(n1). Diffractive lens 42 may be formed from a second material that hasa second index of refraction (n2). In the example of FIG. 4A, the indexof refraction of the lens may be greater than the index of refraction ofthe surrounding material (i.e., n2>n1). This results in the light beingfocused to a focal point.

In particular, incident light 46-3 may pass by the edge of diffractivelens 42. The light may be redirected such that the output light 46-4travels at an angle 48-1 relative to the incident light 46-3. This anglemay be dependent upon the thickness 56 of diffractive lens 42. In theexample of FIG. 4A, thickness 56 is associated with an angle ofdiffraction of 48-1. Diffractive lens 42 in FIG. 4A may have arelatively large thickness and, accordingly, a relatively large angle ofdiffraction 48-1.

In contrast, diffractive lens 42 in FIG. 4B may have a relatively smallthickness and a relatively small angle of diffraction 48-2. As shown inFIG. 4B, a diffractive lens 42 may be formed in a surrounding medium 44.The surrounding material 44 may be formed from a first material that hasa first index of refraction (n1). Diffractive lens 42 may be formed froma second material that has a second index of refraction (n2). In theexample of FIG. 4B, the index of refraction of the lens may be greaterthan the index of refraction of the surrounding material (i.e., n2>n1).This results in the light being focused to a focal point. In particular,the light at the edge of the diffractive lens may be redirected suchthat the output light 46-4 travels at an angle 48-2 relative to theincident light 46-3. This angle may be dependent upon the thickness 58of diffractive lens 42. Because thickness 58 in FIG. 4B is less thanthickness 56 in FIG. 4A, angle 48-2 in FIG. 4B is less than angle 48-1in FIG. 4A.

Diffractive lenses 42 in FIGS. 4A and 4B have the same length and width.However, the length and width of diffractive lenses may also be adjustedto alter the response of incident light 46. The diffractive lenses mayonly redirect incident light that passes within a given distance of theedge of the diffractive lens (e.g., the interface between the twomaterials having different indices of refraction). The given distancemay be approximately one half of the wavelength of the incident light.

This shows how diffractive lenses may be used to redirect incident lightin desired ways. The refractive indices of the lens and surroundingmaterial may be altered to customize the response of incident light.Additionally, the thickness, length, and width, of the diffractive lensmay be altered to customize the response of incident light.

In FIGS. 3A, 3B, 4A, and 4B, diffractive lenses (e.g., diffractive lens42 and diffractive lens 50) are depicted as being formed from a singlelayer of material having a first index of refraction that is surroundedby a surrounding medium having a second index of refraction that isdifferent than the first index of refraction. Because these diffractivelenses have one uniform index of refraction (and therefore onerefractive index difference between the lens and surrounding medium),these types of diffractive lenses may be referred to as single-edgediffractive lenses. These types of diffractive lenses may also bereferred to as single-refractive-index diffractive lenses.

The aforementioned single-edge diffractive lenses may be effective atfocusing or defocusing light at the edges of the diffractive lens. Lightat the center of the diffractive lenses may pass through without beingfocused or defocused as desired. However, light between the center andedges of the diffractive lenses passes through the diffractive lenswithout being focused or defocused. This may not be desirable, asperformance of the lens may be improved if light between the center andedges of the diffractive lens was also focused or defocused.

To better focus light, a diffractive lens may therefore have two or moreportions with different refractive indices. Examples of this type areshown in FIGS. 5A and 5B.

As shown in FIG. 5A, a diffractive lens 62 may be formed in asurrounding medium 44. The surrounding material 44 may be formed from afirst material that has a first index of refraction (n1). Diffractivelens 62 may have first portions 64 formed from a second material thathas a second index of refraction (n2) and a second portion 66 formedfrom a third material that has a third index of refraction (n4). In theexample of FIG. 5A, the index of refraction of the second portion of thelens (n4) may be greater than the index of refraction of the firstportion of the lens (n2) and the index of refraction of the firstportion of the lens may be greater than the index of refraction of thesurrounding material (i.e., n4>n2>n1). This results in incident lightbeing focused towards a focal point. In this arrangement, diffractivelens 62 acts as a convex lens.

Lens 62 (i.e., both portions 64 and 66 of lens 62) may be transparent toincident light. Therefore, some light may pass through the lens withoutbeing focused. For example, incident light 46-1 may pass through thecenter of portion 66 of diffractive lens 62. The corresponding light46-2 on the other side of the diffractive lens may travel in the samedirection as incident light 46-1. In contrast, incident light at theedge of diffractive lens 62 may be redirected due to diffraction. Forexample, incident light 46-3 may pass by the edge of diffractive lens62. The light may be redirected such that the output light 46-4 travelsat an angle relative to the incident light 46-3. In other words, thediffractive lens redirects the light at the edge of the lens usingdiffraction. Additionally, due to the additional refractive indexdifference between portions 64 and 66 of the diffractive lens, lightbetween the edge and center of the diffractive lens may also beredirected. For example, incident light 46-5 may pass by the interfaceof portions 64 and 66 of diffractive lens 62. The light may beredirected such that the output light 46-6 travels at an angle relativeto the incident light 46-5.

The difference in refractive index between each material may be anydesired refractive index difference (e.g., greater than 0.2, greaterthan 0.3, greater than 0.4, greater than 0.5, greater than 0.8, greaterthan 1.0, between 0.2 and 0.5, between 0.2 and 0.8, between 0.2 and 1.0,less than 1.0, less than 0.5, less than 0.3, etc.).

The example of the diffractive lens having two portions in FIG. 5A ismerely illustrative. If desired, the diffractive lens may have threeportions having different refractive indices (as will be shown in FIG.5C), four portions having different refractive indices, five portionshaving different refractive indices, more than five portions havingdifferent refractive indices, etc. Regardless of how many portions arepresent in the diffractive lens, each pair of adjacent portions may havea corresponding refractive index difference. For example, the refractiveindex of each portion may increase proportionally with the distance ofthe portion from the edge (meaning that an edge portion such as portion64 has a lower refractive index than a center portion such as portion66). Said another way, the refractive index of each portion may decreaseproportionally with the distance of the portion from the center.

In the example of FIG. 5A, diffractive lens 62 causes incident light tobe focused towards a focal point. However, this example is merelyillustrative and other embodiments may be used. FIG. 5B shows adiffractive lens with two or more portions having different refractiveindices that defocuses light.

As shown in FIG. 5B, a diffractive lens 72 may be formed in asurrounding medium 44. The surrounding material 44 may be formed from afirst material that has a first index of refraction (n1). Diffractivelens 72 may have first portions 74 formed from a second material thathas a second index of refraction (n3) and a second portion 76 formedfrom a third material that has a third index of refraction (n5). In theexample of FIG. 5B, the index of refraction of the second portion of thelens (n5) may be less than the index of refraction of the first portionof the lens (n3) and the index of refraction of the first portion of thelens (n3) may be less than the index of refraction of the surroundingmaterial (i.e., n5<n3<n1). This results in incident light beingdefocused. In this arrangement, diffractive lens 72 acts as a concavelens.

Lens 72 (i.e., both portions 74 and 76 of lens 72) may be transparent toincident light. Therefore, some light may pass through the lens withoutbeing focused. For example, incident light 46-1 may pass through thecenter of portion 76 of diffractive lens 72. The corresponding light46-2 on the other side of the diffractive lens may travel in the samedirection as incident light 46-1. In contrast, incident light at theedge of diffractive lens 72 may be redirected due to diffraction. Forexample, incident light 46-3 may pass by the edge of diffractive lens72. The light may be redirected such that the output light 46-4 travelsat an angle relative to the incident light 46-3. In other words, thediffractive lens redirects the light at the edge of the lens usingdiffraction. Additionally, due to the additional refractive indexdifference between portions 74 and 76 of the diffractive lens, lightbetween the edge and center of the diffractive lens may also beredirected. For example, incident light 46-5 may pass by the interfaceof portions 74 and 76 of diffractive lens 72. The light may beredirected such that the output light 46-6 travels at an angle relativeto the incident light 46-5.

The difference in refractive index between each material may be anydesired refractive index difference (e.g., greater than 0.2, greaterthan 0.3, greater than 0.4, greater than 0.5, greater than 0.8, greaterthan 1.0, between 0.2 and 0.5, between 0.2 and 0.8, between 0.2 and 1.0,less than 1.0, less than 0.5, less than 0.3, etc.).

The example of the diffractive lens having two portions in FIG. 5B ismerely illustrative. If desired, the diffractive lens may have threeportions having different refractive indices, four portions havingdifferent refractive indices, five portions having different refractiveindices, more than five portions having different refractive indices,etc. Each pair of adjacent portions may have a corresponding refractiveindex difference. For example, the refractive index of each portion maydecrease proportionally with the distance of the portion from the edge(meaning that an edge portion such as portion 64 has a higher refractiveindex than a center portion such as portion 66). Said another way, therefractive index of each portion may increase proportionally with thedistance of the portion from the center.

FIG. 5C is a cross-sectional side view of an illustrative diffractivelens with more than two portions. As shown in FIG. 5C, a focusingdiffractive lens 62 may be formed in a surrounding medium 44. Thesurrounding material 44 may be formed from a first material that has afirst index of refraction (n1). Diffractive lens 62 may have firstportions 64 formed from a second material that has a second index ofrefraction (n2), a second portion 66 formed from a third material thathas a third index of refraction (n4), and a third portion formed from afourth material that has a fourth index of refraction (n6). In theexample of FIG. 5C, the index of refraction of the third portion of thelens (n6) may be greater than the index of refraction of the secondportion of the lens (n4), the index of refraction of the second portionof the lens (n4) may be greater than the index of refraction of thefirst portion of the lens (n2), and the index of refraction of the firstportion of the lens may be greater than the index of refraction of thesurrounding material (i.e., n6>n4>n2>n1). This results in incident lightbeing focused towards a focal point. In this arrangement, diffractivelens 62 acts as a convex lens. The portions of the diffractive lens inFIG. 5C may be concentric (i.e., all of the portions share a commongeometrical center).

In the examples of FIGS. 5A-5C, symmetrical diffractive lenses with twoor more portions are shown. However, the diffractive lenses do not needto be symmetrical. The diffractive lens may instead be asymmetric tocontrol the focusing/defocusing of the light in any desired manner. Thediffractive lenses may be asymmetrical as shown in FIGS. 5D and 5E. Asshown in FIG. 5D, a diffractive lens 62 may be formed in a surroundingmedium 44. The surrounding material 44 may be formed from a firstmaterial that has a first index of refraction (n1). Diffractive lens 62may have a first portion 64 formed from a second material that has asecond index of refraction (n2) and a second portion 66 formed from athird material that has a third index of refraction (n4). In the exampleof FIG. 5A, the index of refraction of the second portion of the lens(n4) may be greater than the index of refraction of the first portion ofthe lens (n2) and the index of refraction of the first portion of thelens may be greater than the index of refraction of the surroundingmaterial (i.e., n4>n2>n1). This results in incident light being focusedtowards a focal point. In this arrangement, diffractive lens 62 acts asa convex lens. Because the diffractive lens is asymmetric, the focalpoint may not be centered underneath the diffractive lens.

The difference in refractive index between each material may be anydesired refractive index difference (e.g., greater than 0.2, greaterthan 0.3, greater than 0.4, greater than 0.5, greater than 0.8, greaterthan 1.0, between 0.2 and 0.5, between 0.2 and 0.8, between 0.2 and 1.0,less than 1.0, less than 0.5, less than 0.3, etc.).

The example of the diffractive lens having two portions in FIG. 5D ismerely illustrative. If desired, the asymmetric diffractive lens mayhave three portions having different refractive indices, four portionshaving different refractive indices, five portions having differentrefractive indices, more than five portions having different refractiveindices, etc. Regardless of how many portions are present in thediffractive lens, each pair of adjacent portions may have acorresponding refractive index difference.

The asymmetric diffractive lens may instead be a defocusing diffractivelens. As shown in FIG. 5E, a diffractive lens 72 may be formed in asurrounding medium 44. The surrounding material 44 may be formed from afirst material that has a first index of refraction (n1). Diffractivelens 72 may have a first portion 74 formed from a second material thathas a second index of refraction (n3) and a second portion 76 formedfrom a third material that has a third index of refraction (n5). In theexample of FIG. 5E, the index of refraction of the second portion of thelens (n5) may be less than the index of refraction of the first portionof the lens (n3) and the index of refraction of the first portion of thelens (n3) may be less than the index of refraction of the surroundingmaterial (i.e., n5<n3<n1). This results in incident light beingdefocused. However, the defocusing is asymmetric due to the asymmetricstructure of diffractive lens 72. In this arrangement, diffractive lens72 acts as a concave lens.

The difference in refractive index between each material may be anydesired refractive index difference (e.g., greater than 0.2, greaterthan 0.3, greater than 0.4, greater than 0.5, greater than 0.8, greaterthan 1.0, between 0.2 and 0.5, between 0.2 and 0.8, between 0.2 and 1.0,less than 1.0, less than 0.5, less than 0.3, etc.).

The example of the diffractive lens having two portions in FIG. 5E ismerely illustrative. If desired, the asymmetric defocusing diffractivelens may have three portions having different refractive indices, fourportions having different refractive indices, five portions havingdifferent refractive indices, more than five portions having differentrefractive indices, etc. Regardless of how many portions are present inthe diffractive lens, each pair of adjacent portions may have acorresponding refractive index difference.

The diffractive lenses of FIGS. 5A-5E each have two or more portionswith different refractive indices. The diffractive lenses may thereforebe referred to as multiple-refractive-index diffractive lenses. Thediffractive lenses of FIGS. 5A-5E also form multiple refractive indexdifferences. For example, in FIG. 5A, the interface between portions 64and surrounding material 44 is a first refractive index difference andthe interface between portions 64 and portion 66 is a second refractiveindex difference. In FIG. 5B, the interface between portions 74 andsurrounding material 44 is a first refractive index difference and theinterface between portions 74 and portion 76 is a second refractiveindex difference. The lenses of FIG. 5A-5E may therefore sometimes bereferred to as multiple-edge diffractive lenses, multiple-portioneddiffractive lenses, compound diffractive lenses, composite diffractivelenses, multipart diffractive lenses, etc.

FIG. 6 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses. Image sensor 16 may include first and secondpixels such as Pixel 1 and Pixel 2. Pixel 1 and Pixel 2 may includephotosensitive regions 82 formed in a substrate such as siliconsubstrate 80. For example, Pixel 1 may include an associatedphotosensitive region such as photodiode PD1, and Pixel 2 may include anassociated photosensitive region such as photodiode PD2. Isolationregions may optionally be included between and/or around PD1 and PD2.The isolation regions may include a metal or metal/dielectric grid, deeptrench isolation or doped portions of substrate 80. Diffractive lenses62 may be formed over photodiodes PD1 and PD2 and may be used to directincident light towards photodiodes PD1 and PD2.

As discussed in connection with FIG. 5A, diffractive lenses 62 may beformed from two or more portions having different refractive indices(e.g., portions 64 and 66). An additional anti-reflective coating(sometimes referred to as a diffractive lens anti-reflective coating)may be formed on one or more surfaces of diffractive lenses 62 ifdesired.

Color filters such as color filter elements 86 may be interposed betweendiffractive lenses 62 and substrate 80. Color filter elements 86 mayfilter incident light by only allowing predetermined wavelengths to passthrough color filter elements 86 (e.g., color filter 86 may only betransparent to the certain ranges of wavelengths). Color filter elements86 may be part of a color filter array formed on the back surface ofsubstrate 80. A respective diffractive lens 62 may cover each colorfilter element 86 in the color filter array. This example is merelyillustrative. If desired, the diffractive lenses may be formed undercolor filter elements 86 such that the diffractive lenses are interposedbetween the color filter elements 86 and photosensitive regions 82.Light can enter from the back side of the image pixels throughdiffractive lenses 62. While in FIG. 6 image sensor 16 is a back-sideilluminated image sensor, image sensor 16 may instead be a front-sideilluminated image sensor if desired. Photodiodes PD1 and PD2 may serveto absorb incident light focused by diffractive lenses 62 and producepixel signals that correspond to the amount of incident light absorbed.

Color filters 86 may include green filters, red filters, blue filters,yellow filters, cyan filters, magenta filters, clear filters, infraredfilters, or other types of filters. As an example, a green filter passesgreen light (e.g., light with wavelengths from 495 nm to 570 nm) andreflects and/or absorbs light out of that range (e.g., the green filterreflects red light and blue light). An example of a color filter arraypattern that may be used is the GRBG (green-red-blue-green) Bayerpattern. In this type of configuration, the color filter array isarranged into groups of four color filters. In each group, two of thefour color filters are green filters, one of the four color filters is ared filter, and the remaining color filter is a blue filter. If desired,other color filter array patterns may be used.

A layer 94 (sometimes referred to as a planarization layer, passivationlayer, dielectric layer, film, planar film, or planarization film) maybe formed over diffractive lenses 62. Planarization layer 94 may beformed across the entire array of imaging pixels in image sensor 16.Layer 94 may have any desired index of refraction (e.g., greater than,less than, or equal to the index of refraction of portions 64 ofdiffractive lenses 62). A second layer 92 may be formed betweendiffractive lenses 62 (e.g., layer 92 may be interposed between the sidesurfaces of adjacent diffractive lenses 62). Layer 92 may have an indexof refraction that is less than the index of refraction of portions 64of diffractive lenses 62 when diffractive lenses are used to focuslight. Alternatively, however, layer 92 may have an index of refractionthat is greater than the index of refraction of portions 64 of thediffractive lenses if the diffractive lenses were used to defocus light.A third layer 90 may be formed under diffractive lenses 62 betweendiffractive lenses 62 and color filters 86. Layer 90 may have anydesired index of refraction (e.g., greater than, less than, or equal tothe index of refraction of portions 64 of diffractive lenses 62). Layers90, 92, and 94 may be transparent and may be formed from any desiredmaterials. Layers 90, 92, and 94 may be formed from the same materialsor different materials. In one possible example, layers 90, 92, and 94may all be formed from the same material and the diffractive lenses maybe embedded within the material. Layers 90, 92, and 94 may sometimes bereferred to as planarization layers, dielectric layers, or claddinglayers. In some cases, one or more of layers 90, 92, and 94 may beformed from air (i.e., an air gap may present be between diffractivelenses 62).

The difference in refractive index between each diffractive lens portionmay be any desired refractive index difference (e.g., greater than 0.2,greater than 0.3, greater than 0.4, greater than 0.5, greater than 0.8,greater than 1.0, between 0.2 and 0.5, between 0.2 and 0.8, between 0.2and 1.0, less than 1.0, less than 0.5, less than 0.3, etc.).

Each portion of diffractive lenses 62 may be formed from any desiredmaterial. It may be desirable for diffractive lenses 62 to betransparent and formed from a material with a higher refractive indexthan the surrounding materials (e.g., layer 92). Each portion of eachdiffractive lens may be formed from silicon nitride (with a refractiveindex of approximately 2.0), from silicon dioxide (with a refractiveindex of approximately 1.45), from silicon oxynitride (with a refractiveindex of approximately 1.8), or any other desired material. In general,each portion of each diffractive lens 62 may have any desired index ofrefraction (e.g., between 1.8 and 2.0, between 1.6 and 2.2, between 1.5and 2.5, between 1.5 and 2.0, more than 1.3, more than 1.6, more than1.8, more than 2.0, less than 2.0, less than 1.8, etc.). Layer 92 mayalso be transparent and formed from a material with any desiredrefractive index (e.g., a lower refractive index than portions 64 ofdiffractive lenses 62). Planar layer 92 may be formed from a materialwith a refractive index between 1.3 and 1.5, between 1.2 and 1.8,greater than 1.3, or any other desired refractive index.

The refractive indices of the portions of diffractive lenses 62 and thesurrounding material (e.g., layer 92) may be selected such that light isfocused by the diffractive lenses towards the photodiodes of the pixels.FIG. 6 shows incident light 46 being focused towards photosensitive areaPD2 by diffractive lens 62, for example. Focusing incident light in thisway may reduce optical cross-talk between pixels. The example of usingfocusing multipart diffractive lenses in image sensor 16 (as in FIG. 6)is merely illustrative. Any of the diffractive lenses shown herein(i.e., in FIGS. 3A, 3B, 4A, 4B, 5A-5E, 7A-7E, 8A, and 8B) may beincorporated into image sensor 16 in the same manner as shown in FIG. 6.

As previously discussed, the refractive indices of the diffractivelenses and surrounding materials, as well as the dimensions of thediffractive lenses, may be altered to customize the response to incidentlight. Additionally, the distance 102 between each diffractive lens maybe altered to change the response of incident light.

In some embodiments, the diffractive lens over each pixel in the pixelarray may be the same. However, in other embodiments different pixelsmay have unique diffractive lenses to further customize the response toincident light.

In the example of FIG. 6, at least one diffractive lens is formed overeach pixel. These diffractive lenses may take the place of otherper-pixel lenses (i.e., the diffractive lenses may be the only per-pixellenses over the pixels in the pixel array). For example, no microlensesmay be present over each pixel that have curved upper surfaces. Nomicrolenses may be present over each pixel that use refraction to focuslight. Microlenses with curved upper surfaces that use refraction tofocus light may optionally be included over each diffractive lens ifdesired.

In the example of FIG. 6, one multipart diffractive lens is formed overeach pixel. This example is merely illustrative. If desired, more thanone diffractive lens may be formed over each image pixel. For example,one or more focusing diffractive lens and one or more defocusingdiffractive lens may be formed over each pixel. Each diffractive lensmay be either a multipart diffractive lens (as in FIG. 5A or 5B, forexample) or may be a single-part diffractive lens (as in FIG. 3A or 3B,for example). All of the diffractive lenses over a pixel may be focusingdiffractive lenses, all of the diffractive lenses over a pixel may bedefocusing diffractive lenses, or there may be both focusing anddefocusing diffractive lenses over a pixel. Similarly, all of thediffractive lenses over a pixel may be multipart diffractive lenses, allof the diffractive lenses over a pixel may be single-part diffractivelenses, or multipart and single-part diffractive lenses may both beformed over a pixel.

Each diffractive lens 62 may have any desired shape. FIGS. 7A-7E are topviews of illustrative diffractive lenses with different shapes. As shownin FIG. 7A, diffractive lens 62 may have a portion 66 with a rectangular(or square) shape and a portion 64 that laterally surrounds portion 66and has a rectangular (or square) outer perimeter. As shown in FIG. 7B,portion 66 of diffractive lens 62 may be formed from a shape with curvededges such as a circle or oval and portion 64 of diffractive lens 62 maylaterally surround portion 66 and have an outer perimeter that is acircle or oval. In the embodiments of FIGS. 7A and 7B, portions 64 and66 have the same shape. This example is merely illustrative. As shown inFIG. 7C, diffractive lens 62 may have a portion 66 (sometimes referredto as inner portion 66) with a different shape than portion 64(sometimes referred to as outer portion 64). As shown in FIG. 7D, thediffractive lens does not have to be a regular shape. FIG. 7D shows anirregularly shaped diffractive lens. Each portion of the diffractivelens may include one or more planar sides, one or more curved sides thatcurve outward, and/or one or more curved sides that curve inward.Finally, as shown in FIG. 7E, the diffractive lens may be split intomore than one section. Portion 64 of diffractive lens 62 may have two ormore separately formed vertical sections or two or more separatelyformed horizontal sections. Similarly, as shown in FIG. 7E, portion 66of diffractive lens 62 may have two or more separately formed verticalsections or two or more separately formed horizontal sections.

In the example of FIG. 6, portions 64 and 66 of diffractive lens 62 havea uniform height (thickness). This example is merely illustrative. Ingeneral, in a multipart diffractive lens (with multiple portions havingdifferent refractive indices) each portion of the diffractive lens mayhave any desired width, height, and thickness. The width, height, andthickness, of each portion can optionally match the respective width,height, and/or thickness of one or more other portions of thediffractive lens if desired.

FIGS. 5A, 5B, 6, and 7 depict multipart diffractive lenses that havedifferent portions having different refractive indices within a singlelayer. This example is merely illustrative. In an alternate embodiment,a multipart diffractive lens may be formed by two stacked layers havingthe same refractive index and different widths. FIGS. 8A and 8B showexamples of this type. In FIG. 8A, a focusing multipart diffractive lens62 is shown that includes a first layer of material 112 that is formedover and overlaps a second layer of material 114. The first and seconddiffractive lens layers 112 and 114 may both have the same refractiveindex (n2) that is greater than the refractive index (n1) of thesurrounding material 44. The underlying layer (114) may have a smallerwidth than layer 112. With this arrangement, light that passes throughthe diffractive lens at a point between the center of the edge of thelens may be focused.

In FIG. 8B, a defocusing multipart diffractive lens 72 is shown thatincludes a first layer of material 122 that is formed over and overlapsa second layer of material 124. The first and second diffractive lenslayers 122 and 124 may both have the same refractive index (n3) that isless than the refractive index (n1) of the surrounding material 44. Theunderlying layer (124) may have a smaller width than layer 122. Withthis arrangement, light that passes through the diffractive lens at apoint between the center of the edge of the lens may be defocused.

FIGS. 8A and 8B depict stacked layers having the same refractive indexas being used to form the multipart diffractive lens. This example ismerely illustrative. If desired, the stacked layers may have differentrefractive indices. Three, four, five, or more than five stacked layerseach having any desired refractive index may be used to form themultipart diffractive lens.

The stacked diffractive lenses of FIGS. 8A and 8B are symmetric. Theseexamples are merely illustrative. As discussed in connection with FIGS.5D and 5E, the diffractive lenses may be asymmetric. FIGS. 9A and 9B arecross-sectional side views of stacked asymmetric diffractive lenses. InFIG. 9A, a focusing multipart asymmetric diffractive lens 62 is shownthat includes a first layer of material 112 and a second layer ofmaterial 114 that are overlapping. The first and second diffractive lenslayers 112 and 114 may both have the same refractive index (n2) that isgreater than the refractive index (n1) of the surrounding material 44.Layer 114 may have a smaller width than layer 112. Layer 114 may beshifted off-center from layer 112, as shown in FIG. 9A.

In FIG. 9B, a defocusing asymmetric multipart diffractive lens 72 isshown that includes a first layer of material 122 and a second layer ofmaterial 124 that are overlapping. The first and second diffractive lenslayers 122 and 124 may both have the same refractive index (n3) that isless than the refractive index (n1) of the surrounding material 44.Layer (124) may have a smaller width than layer 122. Layer 124 may beshifted off-center from layer 122, as shown in FIG. 9A.

FIGS. 9A and 9B depict asymmetric stacked layers having the samerefractive index as being used to form the multipart diffractive lens.This example is merely illustrative. If desired, the asymmetric stackedlayers may have different refractive indices. Three, four, five, or morethan five stacked layers each having any desired refractive index may beused to form the multipart asymmetric diffractive lens.

In FIGS. 8A and 8B, the smaller layer (i.e., the layer with the smallerwidth) in the stacked diffractive lens is below the larger layer. InFIGS. 9A and 9B, the smaller layer in the stacked diffractive lens isabove the larger layer. In general, any number of layers with anydesired widths may be stacked in any desired order to control thedirection of incident light.

In various embodiments, an image sensor may include a plurality ofimaging pixels and each imaging pixel may include a photodiode and adiffractive lens formed over the photodiode. The diffractive lens mayhave a first portion with a first refractive index and a second portionwith a second refractive index that is different than the firstrefractive index.

The first portion may be an edge portion and the second portion may be acenter portion and the first refractive index may be less than thesecond refractive index. The edge portion may laterally surround thecenter portion. The edge portion may be adjacent a material with a thirdrefractive index that is different than the first and second refractiveindices. The third refractive index may be less than the firstrefractive index.

Each diffractive lens may also include a third portion with a thirdrefractive index that is different than the first and second refractiveindices. The second portion may be interposed between the first portionand the third portion and the second refractive index may be less thanthe first refractive index and greater than the first refractive index.The diffractive lens of each imaging pixel may have a planar uppersurface and a planar lower surface. No microlens with a curved surfacemay be formed over the diffractive lens of each pixel. Each imagingpixel of the plurality of imaging pixels may also include a color filterelement interposed between the photodiode and the diffractive lens ofthat imaging pixel.

In various embodiments, an image sensor may include a plurality ofimaging pixels and each imaging pixel of the plurality of imaging pixelsmay include a photosensitive area, a color filter element formed overthe photosensitive area, and a multipart diffractive lens formed overthe color filter element that focuses incident light on thephotosensitive area.

The multipart diffractive lens may include first and second portionshaving respective first and second refractive indices. The secondportion may laterally surround the first portion. The second refractiveindex may be less than the first refractive index. The multipartdiffractive lens may be surrounded by a material having a thirdrefractive index that is less than the second refractive index. Themultipart diffractive lens may include a first layer and a second layerformed over the first layer, the first layer may have a first width andthe second layer may have a second width that is greater than the firstwidth. The first and second layers may have the same refractive indexand may be surrounded by a material with an additional refractive indexthat is less than the refractive index of the first and second layers.No microlens with a curved surface may be formed over the multipartdiffractive lens of each pixel.

In various embodiments, an image sensor may include a plurality ofimaging pixels and each imaging pixel of the plurality of imaging pixelsmay include a photodiode and a diffractive lens formed over thephotodiode. The diffractive lens may have a plurality of portions withrespective refractive indices that increase as a distance of therespective portion from an edge of the diffractive lens increases. Theplurality of portions may be concentric.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. An image sensor comprising a plurality of imagingpixels, wherein at least one of the plurality of imaging pixelscomprises: a photosensitive area; and a defocusing diffractive lensformed over the photosensitive area, wherein the defocusing diffractivelens has a first portion with a first refractive index and a secondportion with a second refractive index that is different than the firstrefractive index, wherein the defocusing diffractive lens is formedadjacent to a solid material with a third refractive index that isdifferent than the first and second refractive indices, wherein thefirst portion of the defocusing diffractive lens forms more than half ofthe defocusing diffractive lens and wherein the second portion of thedefocusing diffractive lens forms less than half of the defocusingdiffractive lens.
 2. The image sensor defined in claim 1, wherein thedefocusing diffractive lens is an asymmetric diffractive lens.
 3. Theimage sensor defined in claim 1, wherein the first refractive index isless than the second refractive index and wherein the second refractiveindex is less than the third refractive index.
 4. The image sensordefined in claim 1, wherein no microlens with a curved surface is formedover the defocusing diffractive lens of the at least one of theplurality of imaging pixels.
 5. The image sensor defined in claim 1,wherein the at least one of the plurality of imaging pixels furthercomprises a color filter element that is interposed between thephotosensitive area and the defocusing diffractive lens.
 6. An imagesensor comprising a plurality of imaging pixels, wherein at least one ofthe plurality of imaging pixels comprises: a photosensitive area; and adiffractive lens formed over the photosensitive area, wherein thediffractive lens comprises a first layer and a second layer formed overthe first layer, wherein the first layer has a first width, wherein thesecond layer has a second width that is different than the first width,wherein the first and second layers have a first refractive index,wherein the first layer has a first center, wherein the second layer hasa second center, and wherein the second center is offset relative to thefirst center.
 7. The image sensor defined in claim 6, wherein thediffractive lens is an asymmetric diffractive lens.
 8. The image sensordefined in claim 6, wherein the diffractive lens is formed in asurrounding material having a second refractive index that is differentthan the first refractive index.
 9. The image sensor defined in claim 8,wherein the first refractive index is greater than the second refractiveindex.
 10. The image sensor defined in claim 8, wherein the firstrefractive index is less than the second refractive index.
 11. The imagesensor defined in claim 6, wherein no microlens with a curved surface isformed over the diffractive lens of the at least one of the plurality ofimaging pixels.
 12. The image sensor defined in claim 6, wherein the atleast one of the plurality of imaging pixels further comprises a colorfilter element that is interposed between the photosensitive area andthe diffractive lens.
 13. An image sensor comprising a plurality ofimaging pixels, wherein at least one of the plurality of imaging pixelscomprises: a substrate; a photosensitive area in the substrate; acladding layer formed over the substrate; and a multipart diffractivelens formed in the cladding layer, wherein the multipart diffractivelens has a first portion with a first refractive index and a secondportion with a second refractive index that is different than the firstrefractive index and wherein the cladding layer has a third refractiveindex that is less than the first and second refractive indices.
 14. Theimage sensor defined in claim 13, wherein the cladding layer is a solidcladding layer.
 15. The image sensor defined in claim 13, wherein themultipart diffractive lens has a center and an edge, wherein the firstportion is formed in the center, wherein the second portion is formed atthe edge, and wherein the second refractive index is less than the firstrefractive index.
 16. The image sensor defined in claim 15, wherein thesecond portion is formed directly adjacent to the cladding layer. 17.The image sensor defined in claim 13, wherein a first difference betweenthe third refractive index and the first refractive index is greaterthan 0.2 and wherein a second difference between the third refractiveindex and the second refractive index is greater than 0.2.
 18. The imagesensor defined in claim 3, wherein the defocusing diffractive lens has acenter and an edge, wherein the first portion is formed in the center,and wherein the second portion is formed in the edge.
 19. The imagesensor defined in claim 18, wherein the second portion is formeddirectly adjacent to the solid material with the third refractive index.